Weightcoated subsea pipeline section

ABSTRACT

Subsea pipe line sections adapted to be welded together into a continuous subsea pipe line are provided with a weight coating which, apart from its weighting function, also serves to protect the pipes. The weight coating comprises a composite material substantially consisting of a granulated iron ore in a matrix of a thermosetting resin, covered by a high strength durable plastic tube.

This application is a continuation of now abandoned application Ser. No.451,205, filed Nov. 30, 1982.

BACKGROUND OF THE INVENTION

The present invention relates to subsea pipe lines for conveying fluidssuch as petroleum from an offshore wellhead. Such pipe lines aregenerally made up of steel pipe provided with a continuous weightcoating, which in addition to its weighting function also serves toprotect the steel pipe against corrosion and mechanical strains.

Conventional weight coating for subsea pipe lines typically consists ofa casing of reinforced concrete surrounding a fibre-glass reinforcedbituminous compound on the external surface of the steel pipe.

A problem encountered with such conventional weight coating is thatcracks tend to develop in the brittle concrete casing owing to shocksand stresses during pipe transport and laying operations, resulting inpieces of concrete flaking off. Since in addition the inner bituminouscoating has a rather poor adherence to the steel pipe surface, smalleror larger areas of the latter will be exposed to the sea water andconsequently subjected to heavy corrosion.

SUMMARY OF THE INVENTION

The present invention provides a subsea pipe line section comprising asteel pipe provided with a continuous weight coating in the form of arelatively thick layer of a composite material consisting of granules ofa relatively heavy material embedded in a plastic matrix, said compositematerial covered by a high strength, durable plastic tube.

The weight coating according to the invention eliminates the problem ofthe conventional concrete casing, since the plastic matrix effectivelybends to the surface of the steel pipe, particularly if a corrosionresistant primer such as a bituminous epoxy resin has been applied tothe steel surface prior to the plastic matrix. Moreover, the weightcoating according to the invention, in contrast to the conventionalconcrete casing, is elastic and practically non-absorbent, thusproviding a strong, ductile and impenetrable seal on the steel pipesurface. The smooth and hard outer surface of the new weight coatingaccording to the invention also effectively solves the increasingproblem of fishing equipment getting entangled on broken parts andreinforcing rods of concrete weight coatings on prior art subsea pipelines. A further advantage of the new weight coating is that it will becapable of preventing excessive discharge of oil in case of a leakageoccurring in the steel pipe, since the combined plastic tube andcomposite material has been found to withstand a pressure as high as 20atm.

The weight coating according to the invention is primarily intended tobe applied as an external casing on the steel pipe. However, if desiredit may as well be applied as a lining on the internal surface of thesteel pipe to protect it against corrosive fluids flowing therethrough.

The plastic matrix will preferably consist of a thermosetting resin suchas epoxy, polyurethane or acrylic resin, while the heavy materialembedded in the plastic resin preferably consists of a heavy metal ore,e.g. an iron ore such as magnetite, hematite or the like, in the form ofparticles or granules.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting examples of subsea weight coated pipe according to theinvention will be explained in detail below with reference to thedrawings in which:

FIG. 1 is a longitudinal sectional view of a pipe line section accordingto the invention provided with an external weight coating,

FIG. 2 is a partial enlarged view of the wall of the pipe shown in FIG.1,

FIG. 3 is a partial, sectional view of the wall of a subsea pipe linesection according to the invention provided with an internal weightcoating, and

FIG. 4 is a portion view of end portions of two pipe line sections ofthe type illustrated in FIG. 3 at a welded junction thereof.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 the numeral 1 is a steel pipe provided with theprotective weight coating according to the invention applied as anexternal casing on the pipe. This casing comprises a plastic tube 4surrounding a relatively thick layer 3 of a composite materialconsisting of a plastic matrix, preferably a thermosetting resin such asepoxy, polyurethane or acrylic resin, in which are embedded particles orgranules of a heavy material, e.g. an iron ore such as magnetite,hematite or the like.

Favourable results have been obtained using a composit materialconsisting of 85-95 percent by weight of magnetite in a matrix of 5-15percent by weight of epoxy resin. In order to enhance strength andelongation properties a certain amount of fibre glass may be added tothe resin. The matrix resin may of course also include any other desiredadditive such as a filler, accelerator, retarder, etc.

Thus, a composite material 3 consisting of 90% magnetite in a matrix ofepoxy resin will provide a weight coating having a specific gravity ofabout 2.8 g/cm³, compared to a conventional concrete weight coatingwhich has a specific gravity of about 2.2 g/cm³.

The outer plastic tube 4 is preferably made from a plastic materialcapable of withstanding severe mechanical stresses caused by impact,shock, tension and compression. A preferred material for the plastictube 4 is cross-linked polyethylene which is also a highly durablematerial.

The internal surface of the outer tube 4 is preferably provided withspaced, radially inwardly directed lugs or protrusions 5 which serve tocentre the outer tube coaxially on the steel pipe 1 with a radialspacing adjusted to the desired thickness of the composite layer 3. Thelugs 5 also serve to anchor the plastic tube 4 in the composite material3 which in turn adheres to the external surface of the steel pipe 1 suchthat relative displacement between the steel pipe 1 and the plastic tube4 is practically impossible.

The required thickness of the composite material 3 will depend on theconditions on the site where the pipe line is to be laid. Generally athickness of between 1-4 cm will be satisfactory. Also the requiredthickness of the outer plastic tube 4 will depend on the siteconditions, 3-20 mm being a typical range.

In order to achieve the best possible corrosion resistance together withan optimal bond between the steel pipe 1 and the composite material 3,the external surface of the steel pipe 1 in a preferred embodiment ofthe invention is coated with a thin, for example 0.3-0.5 mm film 2 of abituminous epoxy resin.

The above described protective weight coating according to the inventionis applied to the steel pipe 1 by pushing the steel pipe 1 into theouter tube 4 with the lugs 5 contacting the steel pipe surface whichpreferably has been sand blasted beforehand and coated with thebituminous epoxy film 2 which is allowed to set before inserting thepipe into the plastic tube, then injecting the non-hardened compositematerial 3 by per se known means into the annulus between the plastictube 4 and the steel pipe 1 to completely fill this annulus, andallowing the composite material to harden into a rigid and at the sametime elastic material having a strong bonding to the surface of thesteel pipe.

Alternatively the ferriferous granules of the composite material 3 maybe separately inserted into the annulus between the pipe 1 and tube 4 ina first step, and then in a second step the plastic matrix resin isinjected as an easy-flowing composition into the annulus to fill up thevoid between the granules therein.

As shown in FIG. 1 the weight coating 3, 4 terminates somewhat short ofthe ends of the steel pipe 1, leaving exposed steel pipe end portions ofsay 10-40 cm. When joining such pipe sections to form a continuous pipeline the steel pipes are butt-welded together and the two preformedhalves of an axially split socket sleeve (not shown) of the samematerials and cross-sectional dimensions as the weight casing 3, 4 areplaced together on the exposed end portions of the joined steel pipes.The sleeve-halves are sealingly and rigidly bonded to the adjacent steelpipe and weightcasing surfaces by injecting an epoxy resin to the socketsleeve inner surfaces which preferably are somewhat roughened orwafer-patterned.

FIG. 3 illustrates an embodiment of the invention in which the weightcoating is applied as a lining on the internal surface of a steel pipe1' instead of as an external casing as in the previous example. Thus, inFIG. 3 the numeral 2' is a primer for example of a bituminous epoxyresin, 3' is a relatively thick layer of the previously described heavycomposite material, 4' is an inner plastic tube of similar material asthe outer plastic tube 4 in the previous example. The plastic tube 4'has external, spaced lugs or protrusions 5' to centre and rigidly anchorthe plastic tube coaxially in the steel pipe. The steel pipe 1' with theinner weight lining 3', 4' is also shown to have an outercorrosive-resistant coating 6 which may be a conventional coating suchas a bituminous epoxy plastic material. Alternatively however, theexternal surface of the steel pipe 1' may be provided with a weightcasing as described in the previous example shown in FIGS. 1 and 2, thedesired unit weight of the total coating then being distributed betweenthe outer casing and the inner lining.

The above described internal weight lining is applied to the steel pipe1' substantially in the same manner as the external weight casing of thesteel pipe 1 according to the previous example. That is, the plastictube 4' is inserted into the steel pipe 1' with its lugs 5' engaging theinternal surface of the steel pipe, preferably after the latter has beensand blasted and coated with a bituminous epoxy primer 2', the compositeresin 3' is inserted into the annulus between the plastic tube 4' andsteel pipe 1' and allowed to harden, or alternatively as before theferronuous granules may be inserted separately into the annulus betweenthe pipe and tube in a first step and the plastic matrix resin in asecond step is injected into the annulus filled with granules.

As to the thicknesses of the respective individual layers 2', 3', 4' ofthe internal weight lining according to the invention, the same valuesapply as stated above for the external layers 2, 3, 4 in the previouscase of the external weight casing. The inner diameter of the internalplastic tube 4' is determined by the desired cross-sectionalthrough-flow area of the pipe line, and the nominal diameter of thesteel pipe 1' is then determined on the basis of the desired thicknessof the weight coating.

When butt-welding pipe sections with internal plastic weight lining ofthe above described type a problem will arise as heat from the weldingoperation may weaken the internal plastic lining in the welding areas.This problem may be solved in the following manner with reference toFIG. 4.

Like in the previous example of the external weight casing the internalweightlining is terminated 10-40 cm short of the ends of the steel pipe1'. When the pipes are to be joined the two internally exposed steelpipe ends are moved by sliding onto a preformed lining sleeve 7 made ofthe same material and having the same cross-sectional dimensions as thepipe lining 3', 4', heat resistant seal rings 8, for example of anasbestos material, having first been placed against the end faces of thelinings of the two pipe sections. The length of the lining sleeve issuch that when the pipe sections are pushed fully against each otherready for welding, the seal rings 8 are in a compressed, fluidtightcondition. The mid-portion of the sleeve 7, which will be situateddirectly opposite the weld seam 9 between the pipe section ends, isformed with a circumferential recess or groove 10 filled with a heatinsulating and resisting material 11. for example burned clay or thelike. Now the welding together of the ends of the two steel pipes 1 maybe performed without any risk of weakening the internal plastic liningwhich thus will extend in a sealed and continuous manner at the pipejunction area.

Although at the present iron ore appears to be the most practical andeconomical material to be used as the weighting component of thecomposite material 3 or 3' of the weight coating according to theinvention, it is within the scope of the invention to utilize othersuitable materials of relatively large specific gravity, e.g. lead-,zinc- or titantium-containing minerals.

It will be appreciated that the use of weight coated pipe line sectionsas disclosed herein is not limited to pipe lines located entirely on thesea bottom. Thus, the pipe line sections advantageously may be used as ariser pipe extending from a subsea wellhead to a production platform orthe like on the sea surface.

What I claim is:
 1. In a subsea pipe line section of the type includinga steel pipe covered with a continuous weight coating, the improvementwherein said weight coating comprises:a relatively thick layer of acomposite material consisting of from 85 to 95% by weight of granulatedheavy metal ore bound and embedded in a matrix of an epoxy, polyester oracrylic thermosetting resin; a protective sheathing in the form of ahigh strength, durable plastic tube covering said layer of compositematerial; and lugs integral with said tube and extending therefrom intosaid layer of composite material and thereby anchoring said tube to saidlayer.
 2. The improvement claimed in claim 1, wherein said weightcoating is in the form of an external casing on the outer surface ofsaid steel pipe.
 3. The improvement claimed in claim 1, wherein saidweight coating is in the form of an internal lining on the inner surfaceof said steel pipe.
 4. The improvement claimed in claim 1, wherein saidweight coating is in the form of an external casing on the outer surfaceof said steel pipe and of an internal lining on the inner surface ofsaid steel pipe.
 5. The improvement claimed in claim 1, furthercomprising a coating of corrosion-resistant primer bonding saidcomposite material to said steel pipe.